In reply to stp:
There are a few reasons for the slowdown recently.
Regarding Intel specifically, it has no competition in the desktop CPU market. AMD has been way out of it for the last 5-6 years polishing the turd that was the Bulldozer architecture. AMD lacked single core performance largely in part of sharing cache between cores for and sacrificing clocks per cycle persuing more cores. Unfortunately the optimisation of applications splitting work between multiple cores for most mainstream applications didn't come as soon as AMD expected.
To add insult to injury, Intel were getting more cpus per silicon wafer because of their more advanced manufacturing techniques (more on this later) making AMDs cost more per chip and cutting into the little profit they could have made with an aggressive pricing strategy.
Anyway, in response to no competition Intel cut funding to it's R & D and abandoned it's 'tick, tock' cycle of development. Also Intel often went back and increased prices of existing CPUs when the latest generation of AMD chips didn't deliver. Hopefully we are seeing a remergence of competition as AMD hired the guy that made AMD their last brilliant chip and the benchmarks for their new Zen architecture look promising enough.
There is little more to this story. The main developments in speed and power efficiency over the years has been generally down to shrinking down the size of each transistor, with the latest step (commercially) Intel took it from 22 nanometer to 14nm. Due to the chemical properties of silicon we're unlikely to get transistors much smaller than around 7nm (this is about 35 atoms long) which is why Intel say that by 2020 their 7nm process will involve a semiconductor other than silicon.
Post edited at 13:37